1. Field of the Invention
The present invention relates to an optical detection apparatus, which detects light from a conveyed sheet, for example, and a sheet processing apparatus having the optical detection apparatus.
2. Description of the Related Art
A sheet processing apparatus for counting and identifying various kinds of sheets has been practically used. A sheet processing apparatus takes in sheets input to an input section one by one, and conveys it to the optical detection apparatus. The optical detection apparatus detects light from a sheet, and obtains an image based on the detected light. Based on the obtained image, a sheet processing apparatus determines the kind, authenticity, and possibility of re-circulation of a sheet. Based on the determination results, a paper processing apparatus sorts and stacks sheets.
On a sheet to be processed by a sheet processing apparatus, fluorescent printed information (first printed information) and phosphorescent printed information (second printed information) are printed. Fluorescent printed information is printed with ink including a fluorescent substance (fluorescent ink). A fluorescent substance is excited by an excitation light (e.g., ultraviolet light) emitted from an excitation light source, and becomes a state to emit light (fluorescence). If the emission of excitation light is interrupted, a fluorescent substance becomes a state not to emit light. Phosphorescent printed information is printed with ink including a phosphorescent substance (phosphorescent ink). A phosphorescent substance is excited by an excitation light (e.g., ultraviolet light) emitted from an excitation light source, and becomes a state to emit light. If the emission of excitation light is interrupted, a phosphorescent substance becomes a state to emit light that is gradually attenuated (afterglow).
A sheet processing apparatus has a fluorescence detector, which emits an excitation light from an excitation light source to a sheet, excites a fluorescent substance, detects fluorescence emitted from a fluorescent substance, and reads fluorescent printed information. A sheet processing apparatus has an afterglow detector, which emits an excitation light from an excitation light source to a sheet, excites a phosphorescent substance, detects afterglow emitted from a phosphorescent substance after the emission of excitation light is interrupted, and reads phosphorescent printed information.
For example, Jpn. Pat. Appln. KOKAI Publication No. 3790931 discloses a technology, which detects fluorescent light and afterglow by a reflective light-receiving system provided in both upstream and downstream of a conveying path on both sides of an ultraviolet lighting unit as an excitation source. In this case, the ultraviolet lighting unit is intermittently lit. In other words, fluorescence is detected when the ultraviolet lighting unit is lit, and afterglow is detected when the ultraviolet lighting unit is turned off.
Jpn. UM Appln. KOKOKU Publication No. 62-2691 discloses a technology, in which an ultraviolet lighting unit and a fluorescence detector are provided in the upstream of a conveying path, and an afterglow detector is provided in the downstream of a conveying path.
According to the technology disclosed in the above Jpn. Pat. Appln. KOKAI Publication No. 3790931, a sheet processing apparatus has two detectors for detecting fluorescence and afterglow. Thus, two processing systems are required for processing obtained images. This increases the system configuration and cost.
Further, when phosphorescent printed information is obtained by using the optical detection apparatus disclosed in the Jpn. UM Appln. KOKOKU Publication No. 62-2691, the time to detect afterglow after interruption of excitation light depends on a speed of conveying a sheet. The amount of attenuation of afterglow increases with time. Thus, when the speed of conveying a sheet is changed, the optical detection apparatus cannot correctly read phosphorescent printed information.
The amount of luminescence of an excitation light source changes with age. The sensitivity of a detecting element of the optical detection apparatus may change with age. Therefore, the optical detection apparatus having a reference fluorescent plate near a reading position is generally used. Such the optical detection apparatus corrects the level of a detected signal based on the level of a reference light detected by the reference fluorescent plate. This increases the accuracy of the optical detection apparatus. However, an excitation light source is not provided in the optical detection apparatus to detect afterglow in the downstream. Thus, even if a reference fluorescent plate is provided, a reference light cannot be detected. As a result, the optical detection apparatus in the downstream cannot be corrected.
Further, when fluorescence and afterglow are obtained by using the optical detection apparatus disclosed in the Jpn. Pat. Appln. KOKAI Publication No. 3790931, a sheet conveying speed cannot be increased. It is necessary to emit an excitation light for a certain period of time for exciting phosphorescent and fluorescent substances. However, an excitation light source repeats turning on and off whenever a sheet passes. Thus, if a conveying speed is increased, the time to emit an excitation light to a sheet is reduced. In other words, the levels of fluorescence and afterglow are changed depending on the conveying speed. Further, as the time to emit an excitation light to a sheet is short, fluorescent and phosphorescent substances may not be sufficiently excited.